The region called the Underdark is an entire world beneath the feet of the surface dwellers of Therafim. Inhabited by monstrous and evil creatures that shun the daylight, the Underdark teems with entire cities and nations of derro, drow, duergar, and mind flayers (illithids). It is also home to even stranger races such as aboleths beholders, and kuo-toas, as well as slaves of just about any creature found in Therafim, sometimes even beyond.

These evil beings battle or trade with each other for resources, magic, and power, forming alliances that collapse when plots unravel or better opportunities come along. Interspersed with the warring city-states are enclaves of gnomes, svirfneblin, dwarves, and other neutral and good folk, who remain isolated or resist encroachment by malign neighbors. There are also other things down there, perhaps entire civilizations it is rumoured that have simply fallen off of the map, ruins maybe… or perhaps they still live, what is known of the Underdark barely scratches the surface.

For surface adventurers, simply entering the Underdark and returning to tell the tale is an accomplishment of note.

Travel in the Underdark

The vast, lightless Underdark is a dangerous place indeed. To nonnative beings, its darkness and tight confines can be insurmountable terrors. To Underdark natives, lights mean danger—either they are lures to bring travelers to some predator, or they mark the place of creatures too powerful to fear attracting attention to themselves. Much of the Underdark was created by volcanism, flowing or dripping water, earth tremors, and mysterious magic that left behind an intense magical radiation. Any of these forces can cause sudden collapses, floods, or falling hazards.

Drinkable water is all too rare, apt to be guarded by predators or already in use by molds or fungi dangerous to other creatures. Some waters are acidic, corroding metal, flesh, or both. Bad air is another problem. Both the native cities and the ongoing voleanism can generate poisonous vapors that drift or creep for miles in the lightless depths.

The Underdark is not a static, unchanging place. Drow, deep gnomes, and other races that dwell in subterranean cities claim the prime territory. Purple worms, umber hulks, xorn, and other creatures burrow through solid rock. Fell races such as beholders and illithids wield strange spells and powers to shape the rocks around them, while the dwarves simply tunnel with skill and determination. Mysterious, unexplained appearances of chambers, caverns, and passages that pulse with strong magical radiation are not unknown. Underdark maps thus soon go out of date. Guides can be helpful, but their loyalty is always a matter of concern.

Travel or teleport magic is usually useless over long distances in the Underdark because it is warped into deadly failure by magical radiation, though it can still prove useful for short journeys within the same open cavern. Portals work in some areas but not in others, and the establishment of new portals is well nigh impossible, since the magic involved attracts formidable enemies very swiftly it is not to be attempted lightly. Like the established trade routes, known portals are always guarded or closely watched.

Supplies are another problem. Only in Underdark cities and a few scattered settlements are shops to be found, and those cities are widely seen as collections of murderers and thieves while any settlement is likely either unwilling to trade, supplied only for their needs, or just as bad or worse than the cities.

The very lack of supplies in the Underdark has created vigorous, ongoing trade. A merchants' caravan may provide the best protection to a visitor to the Underdark. The large amount of carried supplies and fearsome fighting capability, magical and mundane, of a caravan offers safety in unsafe territory. However, care must be taken to avoid unintentionally offering oneself as a slave while employing such a tactic.

Life and Society

The Underdark is a harsh realm where two overwhelming drives rule: survival and the destruction of your enemies. Perpetually dark in most
regions, the Underdark is filled with creatures that long ago developed darkvision or enhanced senses to compensate, often becoming intolerant
of true light as a result of their adaptation. Some places are dimly lit by glowing rocks, luminous crystals, or phosphorescent moss, lichen, and fungi. Bizarre plants are common, and visitors usually find it impossible to identify which are hostile or poisonous without magic or potentially lethal experimentation. The most precious resource is fresh water, since the Underdark has no rain and inhabitants must rely on whatever filters down from the surface. Those who discover water hoard it and protect it with their lives.

Because of the scarcity of certain resources, each city often specializes in producing a few items and trading these with neighbors in peaceful times. A typical trade caravan consists of several dozen heavily armed merchants and soldiers, with two to three patrols sent forward or behind while traveling. Although the tunnels are generally silent, echoes travel far, and a skilled Underdark scout learns to recognize subtle signs of natural animals and lurking threats by their echoes alone. Wealthier cities teleport caravans to their destinations or use existing portals or teleportation nodes to speed travel but no method is truly reliable, and access to a convenient portal is often the cause of lengthy wars between nearby cities.

Cities that develop a reputation for killing or enslaving caravans in peacetime (as opposed to exhibiting cool hostility and rudeness, which are expected) usually find themselves cut off from valuable resources and made easy prey for aggressors.

Major Geographical Features

Because most of the Underdark has been only cursorily surveyed at best, and given that the largest caves are only a few miles long, few geographical features would be considered noteworthy to a surface dweller. Lakes tend to be the largest features, although the nearby stone may dip below the surface of the water, breaking it into quasiseparate regions that can only be identified as the same body of water by the most meticulous cartographer.

Tunnels in the Underdark extend for miles, some ballooning into caverns thousands of feet across, only to shrink to narrow spaces too small for a halfling to squeeze through. The largest cavern halls become representations of the surface in miniature, with hills, valleys, underground rivers, and lakes. In this three-dimensional environment, most races make use of the walls and ceilings of their caverns, accessing the higher levels with natural or magical flight or levitation or wall-crawling mounts such as giant spiders or certain breeds of lizards.

However for all of the areas hazards it is the most naturally rich area in all Therafim for natural materials, rare ores, gems, metals. Rarely are things so easy however, mining camps spring up here and there with many of the inhabitants of the Underdark having mines of their own within their territory. The richest veins are always the most perilous, with few on the upper levels and many deposits nestled away in dark hidden corners of the deepest levels where few wise men would tread. Just because it's dangerous doesn't stop people trying though, and some of the ventures even suceed… luck which in the Underdark is almost certain to run out before long.

Layers of the Underdark
Although it's difficult to map on flat parchment because of its layers and sloping tunnels, the Underdark is divided into three general levels:

The upper Underdark is close to the surface, has considerable interaction (trade, raids, or conquest) with surface races, and is mainly inhabited by drew, beholders, dwarves, mind flayers, svirfneblin, and who knows what else. Water and food are relatively abundant, and adaptation to darkness is mild.

The inhabitants of the middle Underdark tend to see surface races as slaves. These include drow, lone aboleths, cloakers, derro, mind flayers,
svirfneblin, kuo-toas and more. Water and food are difficult to find.

The lower Underdark is incredibly strange, filled with alien societies and bizarre cultures, hostile to any unlike them, ruled by aboleths, cloakers, derro, mind flayers and even more bizarre creatures dwell here. Food and water are very rare, so the races here prey upon each other for survival. Adaptation to darkness is often extreme, with new and peculiar senses appearing in some monsters.

Regional History

The history of the Underdark predates and parallels that of the surface world. Progenitor races arose in the Underdark and died out over time or were slain when their equivalents retreated underground in the face of opposition from new races such as elves and dwarves. Ancient races such as the kuo-toas disappeared from the surface before recorded human or elven history. Evidence of the illithids' origin is scarce, but sages believe that the mind flayers arose at nearly the same time as the kuo-toas or invaded from another plane during that empire's height. The aboleths are also ancient, but the history of their machinations from the lower Underdark has gone unrecorded.

War, conquest, decay, and collapse form the familiar cycle of the Underdark nations. Cities fight each other for riches, resources, or slaves, or out of age-old hatred. Stable empires grow decadent or suffer from numerous and constant skirmishes that bleed away their power. Failing empires collapse, sometimes from within and sometimes prompted by the blades of their enemies. From these broken cities come groups of survivors who find niches where they can scratch out an existence and eventually build new cities. Of course not every settlement in the Underdark is doomed to fail, but here more than anywhere it is a grim and perilous existance.